Heat generation of a semiconductor chip is becoming extremely large along developments in achieving higher integration and higher speed of a semiconductor circuit mounted on a semiconductor chip in recent years, and there is a special configuration for increasing radiation by forming a package unit by attaching a large heat sink to a semiconductor package housing such a semiconductor chip.
FIG. 1 is a schematic diagram of a conventional package unit.
In a package unit 100 shown in FIG. 1, a semiconductor package 30 is mounted on a system substrate 10 by fusing and fixing solder balls 20.
This semiconductor package 30 includes a package substrate 31, a stiffener 33 which is made of stainless steel, copper or the like and is fixed to the package substrate 31 so as to surround a semiconductor chip 32 mounted in the center of the package substrate 31 as well as a peripheral portion of the package substrate 31 for preventing deformation such as warpage of the package substrate 31, and a heat spreader 35 which is typically made of copper and is fixed to the stiffener for receiving heat generated by the semiconductor chip 32 through a thermal junction member 34.
Furthermore, in the package unit 100 shown in FIG. 1, a thermal junction member 40 such as thermal grease or a thermal sheet is put on the heat spreader 35 of the semiconductor package 30, and a heat sink 50 composed of a base 51 and numerous fins 52 is further placed thereon.
A male screw member 61 including a male screw is fixed to the system substrate 10, and meanwhile, a mounting hole 511 for allowing penetration of a female screw member 62 including a female screw is formed on a base 51 of the heat sink 50. A shaft of the female screw member 62 is inserted into a coil spring 63 and the shaft of the female screw member 62 penetrates the mounting hole 511 of the base 51 of the heat sink 50, whereby the female screw of the female screw member 62 and the male screw of the male screw member 61 are screwed together. The coil spring 63 compresses in a space between a head 621 of the female screw member 62 and the base 51 of the heat sink 50 and pushes the heat sink 50 toward the semiconductor package 30, thereby enhancing adhesiveness between the semiconductor package 30 and the heat sink 50 while interposing the thermal junction member 40.
FIG. 2 is an enlarged view inside a circle R illustrated in FIG. 1. It should be noted, however, that illustration of the fins 52 is omitted herein. A problem of the package unit 100 shown in FIG. 1 will be described with reference to FIG. 2.
When using the package unit 100 shown in FIG. 1 for a long time period, the thermal junction member 40 gradually reduces the thickness and expands sideways due to a thermal expansion difference between the semiconductor package 30 and the heat sink 50 caused by power-off of the semiconductor package, thermal distortion of the semiconductor package 30, and constant pressurization by the coil spring 63, whereby a fraction 411 of the thermal junction member 40 may break off and fall out of this package unit 100. Then, if this thermal junction member 40 has electric conductivity, the thermal junction member 40 may short-circuit other circuit components or wiring on the system substrate 10 which are not illustrated herein and thereby causing a malfunction or a failure. Alternatively, if this thermal junction member 40 has an electrical insulation property, the thermal junction member 40 may cause a contact failure when adding or replacing other circuit components onto the system substrate.
To avoid such a problem, Patent Documents 1 and 2 disclose ideas to dispose a thermal compound or grease which is one type of the thermal junction member inside a flange or in a space surrounded by an elastic member so as to prevent leakage of the thermal compound or grease to the outside.
Meanwhile, Patent Document 3 discloses an idea to dispose a spacer between a base corresponding to the system substrate shown in FIG. 1 and a heat sink in order to prevent application of excessive pressing force to a semiconductor chip and the like.    Patent Document 1: Japanese Unexamined Patent Application Publication No. Hei 4(1992)-85861    Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2000-332169    Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2001-168562